Graphene typically refers to a material having less than about 10 graphitic layers. The graphitic layers are characterized by an ‘infinite’ two-dimensional basal plane having a hexagonal lattice structure and various edge functionalities, which may include, for example, carboxylic acid groups, hydroxyl groups, epoxide groups and ketone groups. Graphene nanoribbons are a special class of graphene, which are similarly characterized by a two-dimensional basal plane, but with a large aspect ratio of their length to their width. In this regard, graphene nanoribbons bear similarity to carbon nanotubes, which have a comparable large aspect ratio defined by one or more layers of graphene sheets rolled up to form a cylinder.
Graphene nanoribbons possess a number of useful properties, including, for example, beneficial electrical properties. Unlike carbon nanotubes, which can be metallic, semimetallic or semiconducting depending on their chiral geometry and diameter, the electrical properties of graphene nanoribbons are governed by their width and their edge configurations and functionalization. For example, graphene nanoribbons of less than about 10 nm in width are semiconductors, whereas similar graphene nanoribbons having a width greater than about 10 nm are metallic or semimetallic conductors. The edge configurations of graphene nanoribbons having an “armchair” or “zigzag” arrangement of carbon atoms, along with the terminal edge functional groups, are also calculated to affect the transmission of electron carriers. Such “armchair” and “zigzag” arrangements are analogous to those defined in the carbon nanotube art. In addition to the aforesaid electrical properties, graphene nanoribbons maintain many of the desirable mechanical properties that carbon nanotubes and graphene sheets also possess.
Various methods for making graphene sheets are known, including, for example, adhesive tape exfoliation of individual graphene layers from graphite, chemical-based exfoliation of graphene layers from graphite, and chemical vapor deposition processes, each process providing on the order of picogram quantities of graphene. Several lithographic and synthetic procedures have been developed for producing minuscule amounts of graphene nanoribbons. Microscopic quantities of graphene nanoribbons have been produced by partial encapsulation of carbon nanotubes in a polymer, followed by plasma etching to longitudinally cut the carbon nanotubes. Upon removal of the polymer, graphene nanoribbons are formed. MWNTs have also been non-selectively longitudinally opened by intercalation and reaction with lithium in liquid ammonia solvent, resulting in exfoliation to produce multilayered graphitic structures such as partially opened MWNTs, graphene flakes, and graphene nanoribbons terminated with hydrogens. Macroscopic quantities of graphene nanoribbons have also been produced by a chemical vapor deposition process. Graphene nanoribbons prepared by these processes are typically characterized by multiple graphene layers with a kinked morphology and irregular atomic structure.
In view of the foregoing, more efficient methods for preparation of macroscopic quantities of graphene nanoribbons would be of exceptional benefit. In particular, facile methods to prepare graphene nanoribbons with a more regular atomic structure than are currently available would represent a significant advance in the art.